t4205: unfold across multiple lines

Tests in t4205 test the following:

  git log --format='%(trailers:unfold)' ...

By ensuring the multi-line trailers are unfolded back onto the same
line. t4205 only includes tests for 2-line trailers, but `unfold()` will
fail for folded trailers on 3 or more lines.

In preparation for adding subsequent tests in t6300 that test similar
behavior in `git-for-each-ref(1)`, let's harden t4205 (and make it
consistent with the changes in t6300) by ensuring that 3 or more
line folded trailers are unfolded correctly.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4205-log-pretty-formats.sh

@@ -544,7 +544,7 @@ Signed-off-by: A U Thor
 EOF
 
 unfold () {
-	perl -0pe 's/\n\s+/ /'
+	perl -0pe 's/\n\s+/ /g'
 }
 
 test_expect_success 'set up trailer tests' '